Formation damage involves undesirable alteration of the initial characteristics of a producing formation, typically by exposure to drilling fluids and completion fluids and in some cases during the production phase of the well. The adsorption of additives present in the drilling or completion fluids to the rock or the invasion of solid particles from the drilling fluids tends to decrease the effective pore volume and effective permeability of the producible formation in the near-wellbore region. There may be at least four possible mechanisms at work. First, solid particles from the drilling fluid may physically plug or bridge across flowpaths in the porous formation. Second, when water contacts certain clay minerals in the formation, the clays typically swell, thus increasing in volume and in turn decreasing the pore volume. Third, chemical reactions or fluids incompatibility between the drilling fluid or completion fluids or intervention fluids and the formation fluids or the rock and fluids may precipitate solids or semisolids that plug pore spaces. Four, rock wettability alteration by fluids pumped downhole. Another mechanism of formation damage is the organic and inorganic deposits in the near wellbore region, either by reaction between fluids or during the production phase of the wells.
Reduced hydrocarbon production can result from reservoir damage when a drilling mud deeply invades the subterranean reservoir. It will also be understood that the drilling fluid, e.g. oil-based mud (OBM), is deposited and concentrated at the borehole face and partially inside the formation. Many operators are interested in improving formation clean up and removing the invert emulsion filter cake or plugging material or improving formation damage after drilling into reservoirs with OBMs.
Drilling fluids used in the drilling of subterranean oil and gas wells along with other drilling fluid applications and drilling procedures are known. In rotary drilling there are a variety of functions and characteristics that are expected of drilling fluids, also known as drilling muds, or simply “muds”.
Drilling fluids are typically classified according to their base fluid. In water-based muds, solid particles are suspended in water or brine. Oil can be emulsified in the water which is the continuous phase. Brine-based drilling fluids, of course are a water-based mud (WBM) in which the aqueous component is brine. Optionally, a water-based mud may be free or essentially free from oil.
Oil-based muds (OBM) are the opposite or inverse. Solid particles are suspended in the oil, and water or brine is emulsified in the oil and therefore the oil is the continuous phase. Oil-based muds can be either all-oil based or water-in-oil macroemulsions, which are also called invert emulsions. In oil-based mud, the oil may consist of any oil that may include, but is not limited to, diesel, mineral oil, esters, paraffin, or alpha-olefins. OBMs as defined herein also include syntheticbased fluids or muds (SBMs) which are synthetically produced rather than refined from naturally-occurring materials. SBMs often include, but are not necessarily limited to, olefin oligomers of ethylene, esters made from vegetable fatty acids and alcohols, ethers and polyethers made from alcohols and polyalcohols, paraffinic, or aromatic, hydrocarbons alkyl benzenes, terpenes and other natural products and mixtures of these types.
U.S. Pat. No. 7,134,496 relates to methods of removing an invert emulsion filter cake after a drilling process using a single phase microemulsion. U.S. Pat. No. 8,091,645 concerns in-situ fluid formation for cleaning oil- or synthetic oil-based mud. U.S. Pat. No. 8,091,646 involves single phase microemulsions and in-situ microemulsions for cleaning formation damage. All of these U.S. patents are assigned to Baker Hughes and are incorporated by reference herein in their entirety.
It would be desirable if compositions and methods could be devised to aid and improve the ability to clean up formation damage, oil-based filter cake, and near wellbore damage and to remove it more completely and easily, without causing additional damage to the formation.